The database is about 100gb. I seem to be having a problem with system
resources that is causing new connections to the database to not happen
during peak times. Last time this occurred, I increased the processes
parameter (from 300 to 400) and this alleviated the problem at the time.
But now it happened again. I just restarted the database and all is
well for the time being, but I want to figure out what's happening.

There are no errors in the alert log. The listener.log shows repeated
entries similar to this (I substituted actual names with <>'s):

Metalink research for the TNS 510 error (Note:171636.1) indicates there
is a system resource issue. My problem is that I'm pretty green on
Windows ("unix weenie, am I" -Yoda) and don't know where to look to see
what's being used up. Task manager doesn't show any constraints with
memory or CPU.